1. Field of Invention
The present invention is related to semiconductor nonvolatile memory and in particular to program and program verify for twin MONOS flash memories.
2. Description of Related Art
In MONOS flash memory devices, data is stored as electrons in the nitride region of an oxide-nitride-oxide (ONO) composite layer under a control gate. The presence of electrons in the nitride region increases the threshold of the device. An erased cell with a logical “1” stored has few or no stored electrons in the nitride region, and a programmed cell with a logical “0” stored has a fixed range of electrons in the nitride region. In conventional MONOS memories, the program operation is interrupted by program verify cycles in order to control the number of electrons in the nitride region. A state diagram of prior art for a program operation with program verify is shown in FIG. 1. In the first program setup 101 is needed to startup the charge pumps and setup the voltages needed for the program operation. The selected memory cell is subjected to program voltage conditions during the program pulse step 102. After a fixed time, the threshold of the memory cell is tested in the program verify step 103. If the threshold of the memory cell is greater than the reference threshold, then the memory cell is deemed to have been programmed, and program is complete 104. Otherwise if the threshold of the selected memory cell is not high enough, then the memory cell is placed back into the program state 102.
FIG. 2a gives an example of voltage conditions of a MONOS memory cell of prior art for program and for program verify of prior art in FIG. 2b. The memory cell is composed of a control gate 202, a source 201 and a drain 203. Electrons are stored in the nitride region 204 under the control gate 202. It should be noted that the voltages that are shown are an example only. Actual voltages depend on many specifications such as program speed, oxide thickness, and memory cell dimensions. For CHE (channel hot electron) injection program, a voltage of approximately 10V is applied to the control gate 202 and another high voltage of approximately 5V is applied to the drain 203 with the source 201 grounded.
Referring to FIG. 2b, program verify is very similar to a read operation, in that one diffusion will be measured with respect to a reference to determine the memory state. The control gate 202 is biased to approximately 2V, the drain 201 is biased to approximately 1V and the source 203 is biased to 0V. Whenever there is a transition between program and program verify states, it is necessary to swap the source and drain voltages and to lower the drain 203 voltage from 5V to 0V. If another program cycle is necessary, the drain 203 is raised to 5V again. This is an inefficient usage of charge, because extra current is needed to raise and lower the drain voltage between program and program verify cycles. When the drains of many memory cells are connected to a single highly capacitive bit line, the transition time between program and program verify increases. This increased transition time increases the overall program operation time.
FIG. 3 shows a prior art dual storage MONOS memory device (called NROM) in which there are two memory storage sites 304 and 305 within one memory cell, described in U.S. Pat. No. 6,011,725 (Eitan), which is directed to a method of read called “reverse read”. The diffusion 303, which is closest to the selected memory storage region 305, becomes the lower voltage or source whereas the diffusion 301, which is opposite the selected memory storage region 305, becomes the higher voltage or drain. The drain voltage is higher than the source voltage in order to create a depletion region into the substrate and thus “override” the charge that may be stored in the unselected memory storage region, if it is in the high threshold “0” memory state. This type of NROM memory cell can only operate in reverse read mode, because a higher voltage is needed on the unselected memory storage side to override the unselected memory channel. If the device were to be read in the forward direction, then the higher drain voltage would override the selected memory storage side, and the cell would always be sensed to be in a low Vt “1” memory state.
Another prior art dual storage MONOS device is described in U.S. Pat. No. 6,248,633 called the twin MONOS cell and shown in FIG. 4a. In this type of memory cell there are two extra side wall polysilicon control gate structures 406 and 407 in addition to the word gate 402 and two diffusions 401 and 403. Unlike the control gate 302 of FIG. 3, the word gate 402 of FIG. 4a does not have memory nitride storage regions underneath itself. Instead, the memory storage regions lie underneath the side wall polysilicon control gates 406 and 407. As shown in FIG. 4a, two side wall polysilicon gates between two adjacent memory cells are electrically connected together to define one equivalent control gate. Because the additional control gates 406 and 407 provide another level of flexibility, the twin MONOS cell can be easily read in both the reverse and forward directions. The channel underneath of the unselected nitride storage site can be overridden by increasing the voltage of the associated control gate to a voltage which is some delta above the highest possible threshold voltage (Vcg override). Although the twin MONOS cell is able to read in both directions, the forward read has slower read performance, due to lower cell current, smaller threshold margins, and limited voltage range. FIG 4b shows the relationship of drain voltage v.s. the threshold of the selected nitride region for memory nitride channel lengths of >50 nm and <50 nm. It can be seen that during forward read, the high Vt cell (“0”) suffers degradation of threshold at higher drain-source voltages. This effect becomes more severe for shorter channel lengths. Thus it is desirable to keep the drain voltage to lower than approximately 0.3˜0.5V during sensing in order to maintain reasonable threshold margin between the “1” and “0” cell.
FIG. 4c is a schematic representation of the twin MONOS cell array in the diffusion bit configuration. Each memory cell consists of one word gate, two control gate halves, under which each control gate half is one nitride storage region, and two diffusion halves. In this array, memory cells are arranged in rows and columns, in which word gates are horizontally connected together by word lines WL[0-1], and bit diffusions are vertically connected together by bit lines BL[0-3], and control gates are vertically connected together by control lines CG[0-3]. Control lines CG[0-3] and bit lines BL[0-3] may run atop of one another and have a coupling capacitance of about 30%.
For high bandwidth program applications, it is desirable to program many memory cells in parallel. If many bit lines and control lines are need to be charged and discharged between program and program verify cycles, the voltage and current requirements of the charge pumps and voltage regulators will be very high, which impacts power and overall program time. In multi-level storage memories especially, the number of program and program verify cycles is greater in order to have tighter control between threshold states. Thus it is desirable to minimize the voltage transitions between program and program verify.